A common type of portable heater for temporary use, for example at construction sites or remote work sites in the oil and gas industry, is an indirect fired heater. In this instance, the heated air is clean and free of moisture and contaminated fumes so as to be suitable for areas where persons are working.
U.S. Pat. No. 3,388,697 by Muckelrath and U.S. Pat. No. 4,729,365 by Mutchler disclose examples of indirect fired heaters. In each instance a housing of the heater locates separated heated air and combustion passages which are arranged for exchanging heat with one another. A burner assembly is provided including various components such as fuel lines and a combustion air intake blower opposite from a heated air outlet of the housing. In this instance the burner assembly and related components remain substantially unheated. When used in particularly cold climates, the fuel can gel or other problems may arise from the cold temperatures which result in poor performance of the heater.
U.S. Pat. No. 6,186,138 by Hybertson discloses further example of a portable heater in which a duct is arranged to recycle a portion of the heated air back to the heating air inlet to further heat the air. Although the temperature of the heated air can be better controlled, the burner assembly and related components remain substantially unheated which can result in poor performance of the heater in colder climates as described above.